The aerodynamic loadability of components of a fluid-flow machine, for example fans, compressors, pumps and blowers, is limited by the growth and the separation of profile boundary layers on the blade surfaces and side-wall boundary layers forming on the hub and the casing.
For fans, compressors, pumps and blowers, the state of the art only restrictively provides concepts for the internal guidance of a fluid drawn off at particularly favorable locations and the re-introduction of this fluid into the main flow path at again particularly favorable locations.
The state of the art mostly shows solutions in which a drawn-off fluid quantity is permanently removed from the main flow path of the fluid either by making use of an existing pressure difference or by means of an external pump. This is accomplished either at the axial gap between two blade rows or on surfaces of the blade row itself. Solutions also exist in which the fluid is supplied at the axial gap or to a blade row from an external source.
Some solutions are known in which continuous fluid circulation takes place only on a single blade, for example a rotor blade, with fluid being drawn off from the surface of the blade and re-introduced at the same blade in the blade tip area.
Other concepts known from the state of the art provide for a non-continuous re-circulation of fluid from the rearward to the forward stages of a compressor in order to influence stage de-tune during part-load operation. In these cases, the exchange of fluid is restricted to the axial gaps between the blade rows of the fluid-flow machine.
Other state-of-the-art solutions provide for continuous fluid circulation between different blade rows of a compressor. In these cases, the existing pressure difference is used to remove fluid from a downstream blade row or a downstream axial gap and to re-introduce it at an upstream blade row.
Extraction of fluid on rotors and stators and its transfer to a location outside the flow paths of the fluid-flow machine is shown in U.S. Pat. Nos. 2,720,356, 5,904,470, EP 1 013 937 A2 and DE 1 815 229 A.
Continuous fluid circulation within individual rotor blades is known from U.S. Pat. No. 5,480,284.
Continuous re-circulation of fluid between axial gap and blade row is known from Specification DE 1 428 188 A, while re-circulation from blade row to blade row is shown in U.S. Pat. Nos. 2,749,027, 2,933,238 and U.S. Pat. No. 2,870,957.
The solutions known from the state of the art are characterized by a variety of considerable disadvantages.
Those of the existing concepts which are intended to achieve additional stabilization of the flow in the fluid-flow machine by boundary layer extraction or fluid introduction completely neglect the aspect of a circulation of secondary fluid quantities between the surfaces of different blade rows of the fluid flow machine. Fluid is mostly removed permanently from the main flow path or supplied from an external source—in some cases even by the input of additional energy.
Both an additional energy input and a loss in mass flow will impair the thermodynamic process of the overall system surrounding the fluid-flow machine. Such overall systems include, for example, gas turbines, aircraft engines, power stations and the like. Some concepts make use of a recirculation from blade row to blade row, but with each blade row being limited to either the removal or the supply of fluid.
None of the existing concepts provides for bi-functional flow control on one and the same blade row, i.e. a combination of fluid removal and fluid supply and, hence, a highly effective combination of boundary layer extraction and fluid introduction. An integrated circulation system which provides for recurrent bi-functional flow control via several stages of a fluid-flow machine does not exist either.